The present invention relates to communication systems having optical amplifiers. More particularly, the present invention relates to a system and method for shutting off an optical energy source in an optical communication system in order to protect service personnel repairing a system failure who might otherwise be exposed to harmful optical energy emissions. The present invention is particularly well suited for use in a wavelength division multiplexed (WDM) optical communication system having node distributed intelligence.
Optical communication systems are a substantial and fast-growing constituent of communication networks. The expression xe2x80x9coptical communication system,xe2x80x9d as used herein, relates to any system which uses optical signals to convey information across an optical waveguide medium. Such optical communication systems include, for example, telecommunications systems, cable television systems, and local area networks (LANs).
Optical communication systems in all their forms are currently being challenged by dramatically increasing capacity demands. Current capacity, which is a function of existing waveguide media, is routinely exceeded by an ever increasing traffic of telephone, facsimile, computer Internet, and video data. In theory, this limited system capacity might be expanded by installing additional waveguide media. However, obtaining the necessary rights of way and installing the additional waveguide media are so costly that such expansion is impractical in many instances.
Thus, WDM optical communication systems are currently being incorporated into existing waveguide networks to increase capacity. In a WDM system, a plurality of optical communication signals are carried over a single waveguide, each signal being assigned a particular wavelength. U.S. Pat. Nos. 5,504,609; 5,532,864 and 5,557,439, the disclosures of which are incorporated herein by reference, teach several basic aspects of successful, contemporary WDM systems.
The use of optical amplifiers in WI)M systems to directly and simultaneously amplify a plurality of optical communication signals make WDM systems particularly useful in long distance optical networks. Optical amplifiers are commonly formed by the combination of a section of doped, or xe2x80x9cactive,xe2x80x9d fiber and an optical energy source, typically a pump laser. The active fiber containing a fluorescent substance, generally a rare-earth dopant, accepts energy from the optical energy source, and transfers a portion of the optical energy to an information bearing optical communication signal, or a plurality of optical communication signals traversing the active fiber. The material composition and operation of active fiber amplifiers, is well documented; for example by Bjarklev, Optical Fiber Amplifiers: Design and System Application, (Artech House, Norwood, Mass.), c.1993 and Erbium-Doped Fiber Amplifiers, (John Wiley and Sons, Inc., N.Y.) c. 1994, the disclosures of which are incorporated herein by reference. As used in the context of the present invention, the term xe2x80x9cactive fiber amplifier,xe2x80x9d is broadly construed to cover the entire class of devices, typically comprising a section of active fiber and an optical energy source, without regard to the particular composition of the active fiber or the exact structure and operating characteristics of the optical energy source. Furthermore, for the sake of simplicity throughout the subsequent description of the present invention, the entire, complex interaction between the optical energy source and the active fiber section, whereby optical energy is transferred from the optical energy source to the information bearing optical signal is referred to as the optical energy source xe2x80x9cdrivingxe2x80x9d the optical amplifier.
Unfortunately, the light wavelengths at which many conventional optical energy sources operate are hazardous to the human eye. This well known fact presents optical communication system designers with the challenge of incorporating a mechanism and/or a protocol for shutting off optical energy sources upon detection of a system fault. A xe2x80x9csystem fault,xe2x80x9d as used in describing the present invention, is any mishap or condition which subsequently requires service personnel to intervene at a fiber level within the optical communication system. Typical system faults include a break or dislocation in the waveguide media, an optical amplifier failure, or other event requiring system element replacement.
Previous attempts to address the problem of shutting down the optical energy sources in an optical communication system following detection of a system fault have met with limited success. More importantly, these early attempts severely underperform when compared to what may be accomplished with increasing xe2x80x9cintelligentxe2x80x9d optical communication systems.
Referring to FIG. 1, a simplified optical communication system is shown comprising a West terminal 1, an East terminal 2, and intermediate optical line amplifiers 3, 4, and 5, comprising amplifier nodes 3E, 4E, and 5E arranged along an xe2x80x9cEastxe2x80x9d running waveguide transmitting optical communication signals in a West-to-East direction, and amplifier 3W, 4W, and 5W arranged along a xe2x80x9cWestxe2x80x9d running waveguide transmitting optical communication signals in a East-to West direction. (As used in the present invention the terms xe2x80x9ctransmit,xe2x80x9d and xe2x80x9ctransmissionxe2x80x9d are used to describe the processes of placing an optical signal in a waveguide, the physical movement of the optical signal via the waveguide, and/or the removal of the optical signal from the waveguide). Amplifiers 3E and 3W form intermediate amplifier 3, amplifiers 4E and 4W form intermediate amplifier 4, and amplifies 5E and 5W form intermediate amplifier 5. If a break in the paired East/West waveguides is assumed between intermediate amplifiers 3 and 4, the limitations of conventional safety shut down systems and methods are readily manifest.
Since each intermediate amplifier nominally includes paired amplifier nodes, such as (3E, 3W) and (4E, 4W) in FIG. 1, some early optical communication systems, upon directly detecting the loss of the East running optical communication signal at node 4E, for example, would shut off the optical energy source in the 4W amplifier node. With the resulting absence of an optical communication signal from 4W, 3W would shut off the optical energy source in 3E. Thus, no potentially harmful emissions from amplifier nodes 3E and 4W would escape the waveguide break. While this system quickly resolved the safety hazard, it also placed the optical communication system in a undesirable state. For example, the operational status of intermediate node 5 could not be determined once the system was shut down at intermediate nodes 3 and 4. Further, optical communication systems including such a shut down mechanism required node by node re-initiation of the system, since the intermediate amplifiers could not discriminate between the re-initiation of the optical communication signal and noise, such as a spontaneous optical amplifier emission.
Later attempts were made to remedy these problems, such as in U.S. Pat. No. 5,355,250 (the ""250 patent). The ""250 patent teaches a system which shuts off an optical energy source based on the detected magnitude of an optical communication signal at the input of an optical amplifier. Following detection, the sampled magnitude is compared to a predetermined reference value, and upon failing to meet this value the drive circuit for the optical energy source is turned off. In the system disclosed in ""250 patent and similar systems, the entire bi-directional loop must be shut down node-by-node in cascade to secure portions of the optical communication system posing a safety threat. This result is explained in greater detail below.
In the system disclosed in the ""250 patent, the entire bi-directional loop, between West terminal 1 and East terminal 2, for the example shown in FIG. 1, would be shut down in a cascade of signal loss detections. Again referring to the exemplary system of FIG. 1 and beginning arbitrarily at node 4E at the time the paired waveguides are broken between intermediate amplifiers 3 and 4, the loss of the optical communication signal is detected at node 4E and the optical energy source driving the optical amplifier at node 4E is turned off. The resulting absence of an optical signal at the output of node 4E is subsequently detected at SE which shuts off the optical energy source driving the optical amplifier at node 5E. This process continues until the cascade of optical communication signal xe2x80x9cfailuresxe2x80x9d propagates around the loop to reach node 3E. Thus, while only the optical energy between intermediate amplifiers 3 and 4 is capable of escaping at the waveguide break and potentially injuring service personnel, the entire optical communication system is shut down.
Again, the resulting operational status of the overall optical communication system is undesirable. In the forgoing example, there is no safety reason to shut down the optical amplifiers at amplifier nodes 4E, 5E, 5W and 3W. In fact, in a WDM system, like the one disclosed in U.S. Pat. No. 5,555,118, the disclosure of which is incorporated herein by reference, having the ability to add and drop selected channels at one or more intermediate nodes, the loss of every amplifier node in the optical communication system due to a single point system fault unnecessarily limits system performance. As a particular example, if optical signals corresponding to several channels of West-to-East traffic are routinely injected into the East running waveguide it intermediate amplifier 4 for transmission to East terminal 2, there is no reason why amplifier nodes 4E and 5E can not transmit these signals even after a system fault occurring between intermediate amplifiers 3 and 4. Further, even in the absence of added optical signals, the ability to transmit and receive service channel signal(s) between intermediate and terminal nodes unaffected by the system fault is of great value.
The present invention provides a system for shutting down an optical energy source in an optical communication system. The system includes an upstream node and a downstream node. The upstream node transmits a first optical communication signal and a first service channel signal to the downstream node, and receives a second optical communication signal and a second service channel signal from the downstream node. The first and second optical communication signals may consist of a single optical wavelength or a plurality of multiplexed optical wavelengths.
The downstream node comprises, a downstream service channel circuit receiving the first service channel signal from the upstream node, a first downstream optical amplifier receiving the first optical communication signal from the upstream node and amplifying the first optical communication signal, a second downstream optical amplifier amplifying and transmitting the second optical communication signal to the upstream node, a downstream optical energy source adapted to drive at least the second downstream optical amplifier, and a downstream node control processor communicating with the downstream service channel circuit to determine the state of the first service channel signal, determining the level of the first optical communication signal, and communicating with the downstream optical energy source to shut it off upon determining either an alarm state in the first service channel signal, or an alarm level in the first optical communication signal.
The present invention is particularly well adapted to operation in systems wherein the first and second optical amplifiers are multistage amplifiers. In such systems, the node control processor will shut off only those optical energy sources providing potentially harmful optical emissions, without otherwise effecting overall system performance.
In another aspect, the present invention provides a method of shutting down an optical energy source in a wavelength division multiplexed optical communication system. The optical communication system comprises N nodes arranged along first and second optical waveguides. The method comprises the steps of receiving at node Ni a first plurality of optical communication signals and a first service channel signal from node Nixe2x88x921 via the first optical waveguide, amplifying the first plurality of optical communication signals received at node Ni using a first optical amplifier driven by an optical energy source, receiving at node Ni a second plurality of optical communication signals from node Ni+1 via the second optical waveguide, amplifying the second plurality of optical communication signals received at node Ni using a second optical amplifier driven by the optical energy source, determining the state of the first service channel signal received at node Ni, determining the level of the first plurality of optical communication signals received at node Ni; and upon determining one of an alarm state in the first service channel signal and an alarm level in the first plurality of optical communication signals, shutting down the optical energy source.